Electron multiplier with curved resistive secondary emissive coating



March 29. 1966 R. M. MATHESON 3,243,628

ELECTRON MULTIPLIER WITH CURVED RESISTIVE SECONDARY EMISSIVE COATING Filed June 26, 1962 LIGHT/1 32' ,Zg

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j CM MM, 477%61/5) United States Patent 3,243,628 ELECTRON MULTIPLIER WITH CURVE!) RE- SISTIVE SECONDARY EMISSIVE CQATTNG Robert M. Matheson, Lancaster, Pa., assignor to Radio Corporation of America, a corporation of Delaware Filed June 26, 1962, Ser. No. 2%,307 12 Claims. (Cl. 313-103) This invention relates to electron multipliers. One type of electron multiplier, which has found wide commercial use, is the photosensitive electron multiplier or photomultiplier tube. The photomultiplier tube generally comprises a photocathode and one or more secondary electron emissive electrodes, or dynodes, positioned to receive electrons that are either emitted from the photocathode or from another dynode. The dynodes are normally of particular configuration so that the electrons will pass from one multiplying stage to another, and electron multiplication will occur at each stage as the electrons pass through the various stages of the tube. These photomultiplier tubes also include a collector electrode, or anode, from which output signals are taken. Due to the stringent requirements for particular configurations of the various electrodes, particularly the secondary electron emissive dynodes, these tubes are rather complicated to construct and thus are expensive to produce.

Another type of multiplier tube found in the prior art is one in which two resistive, secondary electron emissive coatings are arranged on two parallel substantially straight planes. In this type of structure, a magnetic field is provided to insure that the electrons bombard the oppositely disposed surfaces of the secondary emissive coatings. The coatings are resistive so that, when a potential is applied between the ends thereof, the electrons will move from the negative end thereof toward the more positive end where they are collected.

Another type of multiplier tube is one wherein resistive coatings are placed on two oppositely disposed parallel plates. By means of the resistive coatings, and a potential between the ends of the coatings, the electrons travel a path that extends substantially parallelly between the plates. By applying first positive and then a negative polarity potential to the plates, the electrons travel from one plate to the other and bombard the other, opposite plate where they are multiplied.

Thus, the more conventional electron multiplier structures require particularly shaped multiplier plates or dynodes. A second type requires a magnetic field to insure that the electrons land on the secondary electron emissive surfaces. Still a third type requires the application of a particular alternating polarity potential between the two plates to insure that the electrons land on the oppositely disposed secondary emissive surfaces.

It is an object of this invention to provide a novel electron multiplier tube that is simple and economical to construct.

It is a further object of this invention to provide an improved photosensitive structure utilizing secondary electron emission amplification.

These and other objects are accomplished in accordance with this invention by providing an elongated curved insulating member having an inner surface on which is a 'ice resistive secondary electron emissive means. The longitudinal axis of the insulating member is curved so that electrons will see a positive field, i.e., an adjacent area of the more positive secondary electron emissive means. The secondary electron emissive means is prepared so as to have a relatively high surface resistance so that, when a potential is applied between the two ends of the secondary emissive means, a potential gradient is established to draw the electrons from their point of emission toward a collector electrode. Due to the curved surface of the secondary electron emissive means, the primary electrons bombard the walls of the secondary emissive means, which bombardment produces secondary electron emission. The secondary electrons again bombard the walls prior to the collection of the multiplied electrons.

The invention will be more clearly understood by reference to the accompanying single sheet of drawings wherein:

FIG. 1 is a longitudinal sectional view of a photomultiplier tube made in accordance with this invention;

FIG. 2 is a longitudinal sectional view of an embodiment of this invention;

FIG. 3 is a longitudinal sectional view of an embodiment of this invention; and

FIGS. 4 and 5 are partial longitudinal sectional views of resistive secondary emissive structures which may be used in tubes made in accordance with this invention.

Referring now to FIG. 1 there is shown a secondary electron multiplier 18). The multiplier tube 10 is shown as being sensitive to an input light 11. Thus, the tube 10 is a photomultiplier tube. It should be clearly understood that electron multipliers of types other than those of a photosensitive input may embody this invention.

The secondary electron multiplier tube 10 comprises an evacuated elongated tubular envelope 12 that has a curved longitudinal axis as shown in FIG. 1. It should be understood that the tubular envelope 12 may have any curved shape such as helical, circular, or any other shape which produces one or more curves.

Within one end of the envelope 12 there is provided a photocathode 14. The photocathode 14 may comprise any conventional photoemissive material such as the S11 photoservice described in U.S. Patent Number 2,676,282 to Polkosky issued April 20, 1954 or the multi-alkali photosurface described in US. Patent Number 2,770,561 to Sommer issued November 13, 1956. Also, the tube 10 may be made sensitive to wavelengths other than the visible, e.g., the ultra violet or infrared, by selecting a photocathode 14 that is sensitive to the desired wavelength.

In the other end of the envelope 12 there is provided a collector electrode 16. The collector electrode 16 may comprise a solid collector plate and may be made of material such as nickel. Also, the collector electrode 16 may comprise a mesh screen (not shown) positioned in front of a reflector type electrode as is known in the photomultiplier tube art.

Between the photocathode 14 and the collector electrode 16, and extending continuously on the inner curved walls of the envelope 12, there is provided a resistive coating 18 which has the property of being secondary electron emissive. The coating 18 is prepared to have a high surface resistance, i.e. greater than 10 ohms per square. The coating 18 may be made of a material different from the photocathode 14, e.g. the coating 18 may be made of tin oxide. Also, the coating 18 may be made of the same material as the photocathode 14. Thus, the coating 18 may be prepared, for example, by evaporating antimony and condensing a thin, e.g. 10 to l cm, thick, film of antimony on the inner surface of the envelope wall. Subsequently, the thin film of antimony is reacted with cesium vapor which may be obtained by flashing a cesium pellet (not shown) within the tube. In this example, the resistive coating 18 would be photosensitive and could be continued to cover the end of the envelope 10 with the separate photocathode 14 being omitted. As was explained, the resistive coating 18 may be made of other materials, which are not photosensitive, such as tin oxide. In the latter case, the materials used for the coating 18 would be selected solely for its secondary emissive properties and its resistive properties. In either case, the resistive coating 18 should have the property of high secondary emission along with a high surface resistance.

During tube operation, when the photocathode 14 and the adjacent end of the resistive coating 18 is at a more negative potential (the photocathode 14 and the adjacent end of the coating 18 may be connected together), such as a ground potential, and the collector electrode 16, as well as its adjacent end of the resistive coating 18, are at substantially higher potentials, e.g approximately 1,000 volts, most of the photoelectrons, emitted from the photocathode 14, have a finite transverse emission velocity so that they are accelerated and will strike some area of the resistive coating 18. The collector electrode 16 is preferably biased positive, eg, 100 volts with respect to the adjacent end of the resistive coating 18. Because of the high secondary electron emission properties of the coating 18, the number of secondary electrons arising from the resistive coating 18 are substantially greater in number, and thus multiplied, as compared to the original number of bombarding primary electrons. Because of the curved surface, and the potential gradient, the electrons again are accelerated toward the collector electrode and again strike the secondary emissive coating 18 at a point closer to the collector electrode 16. This process is repeated, and a current, substantially greater than the primary emission photoelectron current is collected by the collector electrode 16.

Due to the fact that the longitudinal axis of the envelope 12 is curved, the primary or photoelectrons, as well as the multiplied secondary electrons, are accelerated toward, the secondary emissive coating 18. Thus, the electrons bombard the resistive secondary emissive coating 18 on the inner surface of the envelope 12 a plurality of times, producing a plurality of multiplying stages, before being collected by the collector electrode 18.

The number of times the electrons bombard the secondary emissive coating 18, i.e. the equivalent number of multiplying stages, depends primarily upon the potential difference between the ends of the coating 18 and upon the distance between the photocathode and the collector electrode. The longer the distance between the photocathode 14 and the collector electrode 18, the greater will be the number of stages, or dynodes. Also, the greater the voltage gradient, the larger the number of stages.

The initial kinetic energy of the emitted electrons, either primary or secondary is of the order of three volts or less. When the energy contributed by the field, i.e. the energy produced by the voltage applied to the resistive coating, exceeds the initial kinetic energy by two or three times, the field effect predominates. Thus, the electron paths may be sharply curved at low velocities, but the electrons will proceed more and more nearly in a straight line after acceleration. When the electrons attain a veloc- 4 ity of the order of 50 to volts, the electron path is substantially independent of the fields and the electrons will then land on the first thing in its path, i.e. the curved resistive coating.

Referring now to FIG. 2, there is shown an embodiment of this invention in which a photomultiplier tube 20 comprises a tubular envelope 22 which forms a complete circle or toroid. The envelope 22 has a continuous coating 24 of resistive secondary emissive material on most of the inner surface thereof. Positioned in the envelope is a transparent support member 26 which supports a photoemissive cathode 28. The transparent support 26 is positioned so as to receive an input light signal 30. Also positioned in the envelope 22 and closely spaced from the p'hotocathode is a collector electrode 32. The collector electrode 22 is substantially parallel to a radius of the toroid formed by the envelope.

Thus, in the embodiment shown in FIG. 2 there exists a large number of multiplying stages or dynodes, than in the embodiment shown in FIG. 1, because of the increased path length between the photocathode 28 and the collector electrode 3-2. It should be understood that even more stages may be produced by making the envelope in the shape of a spiral.

Referring now to FIG. 3 there is shown an embodiment of this invention in which the tube is sinuous, i.e. the direction of the curvature changes. Thus, the tube 36 comprises a tubular envelope 38 in which the longitudinal axis curves, in diiferent directions, several times. In one end of the envelope 38 a photoemissive cathode 40 is provided. Within the other end of the envelope 38 there is positioned a collector electrode 42. Between the electrodes 40 and 42, the various curves of the envelope 38 are coated with a resistive secondary emissive means 44. Any of the materials previously described may be used in the embodiment shown in FIG. 3. The operation of this embodiment will be clear from what has been said heretofore.

In the embodiments shown in FIGS. 1, 2, and 3 equipotential surfaces are produced which are roughly parallel to radius lines drawn from the center of curvature of the envelopes. Because of this configuration, the electrons are attracted toward the collector electrode while bombarding different areas of the resistive secondary emissive coating. Thus, these multiplier tubes do not require a magnetic field to make the electrons land on the secondary emissive surface. Also separate plates with opposite polarity potentials are not required to produce bombardment of the secondary emissive surface. Still further, the stringent requirement of separate dynode configurations frequently found in the prior art is not necessary.

n the embodiments of this invention the secondary emissive coating has been shown as being supported on a curved surface which comprises the inner Wall of the en velope. It should be clearly understood that any similar curved surface, which is curved in the direction between the origin or the primary electron source, and the collector electrode is suitable for use, and the envelope wall is used as a convenient support structure.

Any particular radius of curvature may also be used, as longas there is no straight line path between the point of origin of the primary electrons and the collector electrode. Also, the curvature should be free of discontinuous imperfections and preferably be of a continuously curved configuration.

Referring now to FIG. 4, there is shown an embodiment of th1s invention which comprises a mosaic 56 of minute. conducting secondary emissive elements deposited on a; resistive surface film 58. One of the advantages of this. configuration is that no potential gradient exists within the secondary emissive elements of the mosaic 56. In other words the gradient is in the resistive film 58. One example of a secondary emissive mosaic 56, as Well as a photoemissive mosaic, is the mosaic used in an i con oscope,

type camera tube. Such a mosaic may be made of cesiumactivated globules of oxidized silver, in a known manner. The resistive film may, as an alternative, be made of cesium-activated patches of antimony evaporated through a fine mesh.

Referring now to FIG. there is shown a further embodiment of this invention wherein the resistive secondary emissive coating is in the form of a resistive spiral 60. The resistive spiral 60 may be formed, for example, by tracing, or by means of an evaporating mask (not shown), a helix of a coating having a much lower surface resistance than the inside wall of the tube. For example, for a surface resistance of ohms, and when using a strip 60 having a specific resistance of 1 ohm and a tube radius of 0.4 inch, the strip width becomes about 1.8 mils when the strip width is equal to the spacing between adjacent strips. A 1 ohm resistance may be formed by means of a platinum film that is approximately 10- cm. thick.

Either of the resistive secondary emissive means shown in FIGS. 4 and 5 may be used in the embodiments shown in FIGS. 1, 2, and 3. Thus, this invention provides a novel photomultiplier structure which is simple to construct and is economical to operate in that magnetic fields are not required, varying polarity potential sources are not required, and individual configurations of separate dynode structures are not required.

What is claimed is:

1. An electron multiplier tube adapted to operate without a magnetic field comprising:

(a) an evacuated envelope;

(b) a source of primary electrons within said envelope;

(0) a collector electrode Within said envelope and spaced from said source of primary electrons;

(d) a continuous member having two spaced resistive secondary electrons emissive portions within said envelope and extending from adjacent said source to adjacent said collector electrode;

(e) said two resistive secondary electron emissive portions being similarly curved so that no straight line electron path exists between said source and said collector electrode; and

(f) means for accelerating electrons from one to the other of said resistive secondary electron emissive portions in substantially straight paths having a component in the direction of said collector electrode.

2. An electron multiplier tube adapted to operate without a magnetic field comprising:

(a) an evacuated envelope;

(b) a source of primary electrons within said envelope;

(c) a collector electrode within said envelope and spaced from said source;

((1) a continuous resistive secondary electron emissive member within said envelope and extending from adjacent to said source to adjacent to said collector electrode; said member comprising two opposed resistive secondary emissive surfaces similarly curved in shape in the direction between said source and said collector electrode such that no straight line electrode path exists between said source and said collector electrode; and

(e) means urging electrons between said surfaces in straight line paths having components directed to said collector electrode.

3. An electron multiplier tube adapted to operate without a magnetic field comprising:

(a) an elongated tubular envelope;

-(b) said elongated envelope having a longitudinal axis which is curved to provide at least a part of a toroid;

(c) a photocathode within one end of said envelope;

((1) a collector electrode within the other end of said envelope; and

(e) a resistive secondary electron emissive electrode on the inner wall of said envelope and extending from adjacent said photocathode to adjacent said collector electrode.

4. An electron multiplier tube as in claim 3 wherein said resistive secondary emissive electrode is connected with said photocathode.

5. An electron multiplier tube as in claim 3 wherein the curvature of said longitudinal axis of said envelope changes directions.

6. A multiplier tube adapted to operate without a magnetic field comprising:

(a) an elongated tubular envelope having a curved longitudinal axis;

(b) a coating of resistive secondary electron emissive material on the inner surface of said envelope and extending substantially throughout the entire length of said envelope;

(0) a collector electrode adjacent to one end of said coating; and

(d) a photocathode adjacent to the other end of said coating.

7. A multiplier tube adapted to operate without a magnetic field comprising:

(a) an elongated tubular envelope having a curved longitudinal axis whereby said envelope is in the shape of a toroid;

(b) a collector electrode within said envelope;

(c) a transparent support positioned adjacent to said collector electrode;

(d) a coating of photoemissive material on the side of said transparent support remote from said collector electrode; and

(e) a continuous resistive, secondary emissive coating on the inner wall of said envelope and extending from adjacent to said photocathode substantially throughout said envelope to adjacent to said collector.

S. A photomultiplier tube adapted to operate without a magnetic field comprising:

(a) an elongated tubular envelope having a longitudinal axis;

(b) said longitudinal axis having at least one curve therein;

(c) a photocathode in one end of said envelope;

(d) a collector electrode means in the other end of said envelope; and

(e) a high resistance secondary emissive electrode means on the inner wall of said envelope and extending from adjacent said photocathode to adjacent said collector electrode.

9. A photomultiplier tube as in claim 8 wherein said means includes a spiral resistive coating.

10. A photomultiplier tube as in claim 8 wherein said means includes a resistive coating having a mosaic of secondary emissive particles thereon.

11. A photomultiplier tube adapted to operate without a magnetic field comprising:

(a) an elongated tubular envelope having a longitudinal axis;

(b) said longitudinal axis having at least one curve therein;

(c) a collector electrode means in one end of said envelope; and

(d) a high resistance means on the inner wall of said envelope and extending from adjacent said collector electrode means throughout the balance of said envelope;

" (e) said high resistance means being photoemissive and secondary electron emissive.

12. A photomultiplier tube adapted to operate without a magnetic field comprising:

(a) an elongated tubular envelope having a longitudinal axis;

(b) said longitudinal axis having at least one curve therein;

(c) a collector electrode means in one end of said envelope;

that no straight line electron path exists between the 10 photoemissive portion of said high resistance means and said collector electrode.

References Cited by the Examiner UNITED STATES PATENTS 2,185,172 1/1940 Bruche 313105 2,209,847 7/1940 Rabateau 313105 2,841,729 7/1958 Wiley 313104 OTHER REFERENCES Article by Goodrich and Wiley, Rev. of Sci. Inst. 42, page 846 (1961).

D. J. GALVIN, Primary Examiner. 

1. AN ELECTRON MULTIPLIER TUBE ADAPTED TO OPERATE WITHOUT A MAGNETIC FIELD COMPRISING: (A) AN EVACUATED ENVELOPE; (B) A SOURCE OF PRIMARY ELECTRONS WITHIN SAID ENVELOPE; (C) A COLLECTOR ELECTRODE WITHIN SAID ENVELOPE AND SPACED FROM SAID SOURCE OF PRIMARY ELECTRONS; (D) A CONTINUOUS MEMBER HAVING TWO SPACED RESISTIVE SECONDARY ELECTRONS EMISSIVE PORTIONS WITHIN SAID ENVELOPE AND EXTENDING FROM ADJACENT SAID SOURCE TO ADJACENT SAID COLLECTOR ELECTRODE; (E) SAID TWO RESISTIVE SECONDARY ELECTRON EMISSIVE PORTIONS BEING SIMILARLY CURVED SO THAT NO STRAIGHT LINE ELECTRON PATH EXISTS BETWEEN SAID SOURCE AND SAID COLLECTOR ELECTRODE; AND (F) MEANS FOR ACCELERATING ELECTRONS FROM ONE OF THE OTHER OF SAID RESISTIVE SECONDARY ELECTRON EMISSIVE PORTIONS IN SUBSTANTIALLY STRAIGHT PATHS HAVING A COMPONENT IN THE DIRECTION OF SAID COLLECTOR ELECTRODE. 